The dendritic trees of neurons in the brain's cortex conduct elemental brain functions such as information processing and learning. Research in brain slices has provided a wealth of information about the dendritic mechanisms that could be operating in behaving animals to integrate, and alter the strength of, synaptic inputs from presynaptic sources. Key among these mechanisms are back-propagating action potentials and nonlinear integration of synaptic inputs leading to dendritic spiking, which confer to the dendriti tree a host of local and global signaling possibilities. Numerous alluring models of information processing and learning have arisen as a result of these in vitro findings, but currently almost nothing is known about which of these mechanisms are at work in awake, behaving animals. The present proposal leverages recent technical advances developed by the PI that enable functional imaging of calcium transients with sub-cellular resolution in the hippocampus of head-restrained mice performing spatial behaviors in a virtual-reality interface. Using these methods, the research proposed in this grant application will allow us to bridge two disconnected areas of neuroscience research: studies that characterize the firing patterns of hippocampal neurons in behaving rodents, and experiments that study the mechanisms underlying firing and plasticity in these cells in reduced preparations. Specifically, we aim to determine the behavioral relevance of the fundamental dendritic mechanisms of bAPs and dSpikes in the hippocampus. This will allow testing of models of plasticity which have been developed based on in vitro data, across a wide range of parameter space, to finally establish which learning mechanisms are behaviorally relevant.